1. Field of the Invention
The present invention relates to a method of manufacturing a superconducting cable. More particularly, the present invention relates to a method of manufacturing a superconducting cable to ensure contraction allowance of a cable core for cooling.
2. Description of the Related Art
After having been laid down, a superconducting cable is cooled by a coolant, such as liquid nitrogen in the cable. The outermost layer of the cable is at ambient temperature, whereas the inside of the cable remains at about xe2x88x92200xc2x0 C. Hence, a temperature difference between the inside and outside of the cable is 200xc2x0 C. or more. Metal constituting the cooled cable is subjected to a contraction of about 0.3%; more specifically, the cable is subjected to a thermal contraction of about 30 cm every 100 meters. A conductor of the superconducting cable is formed by stranding a plurality of cable cores. Both ends of the cable are fixed to an interconnection section or a terminal connection section. Hence, if the thus-stranded cable core undergoes contraction, the strand is fastened much tighter. As a result, the cable is subjected to the tensile stress and the side pressure at bending parts, thereby inflicting damage on the superconductor, whose performance is greatly degraded by mechanical stress. For this reason, a mechanism which manages the thermal contraction is required.
The technique as described in Japanese Patent Unexamined Publication No. Hei. 09-134620 has hitherto been known as a technique for managing such thermal contraction. Specifically, an intermediate substance having a large thermal contraction rate is provided in the center of three cable cores, and the three cables are stranded with the intermediate substance interposed therebetween. The strands of three cable cores change in diameter by virtue of thermal contraction of the intermediate substance, thereby managing thermal contraction.
The above-described technique requires use of the intermediate substance having a large thermal contraction rate in addition to the cable cores, thus increasing the number of components.
An object of the present invention is to provide a method of manufacturing a superconducting cable which can manage thermal contraction without integrating another member with cable cores.
The present invention achieves the object by temporarily providing a spacer between cable cores so that the cores can be housed in a thermally insulated pipe in a slacked state.
The present invention provides a method of manufacturing a superconducting cable comprising:
providing spacers in a plurality of cores at the time of stranding of the cores; and
removing the spacers before the stranded cores are housed in a thermally insulated pipe, and housing the cores into the thermally insulated pipe while the strands are held in a slacked state.
When a plurality of cores; for example, three cores, are stranded, spacers are interposed between the cores. The cores are stranded at intervals corresponding to the thicknesses of the spacers. In case that the thus-stranded cores are housed in the thermally insulated pipe, the spacers are removed and the cores are housed in the pipe while slack corresponding to the thicknesses of the spacers is maintained. At this time, if the thicknesses of the spacers are optimized, it is possible to easily manufacture of three cores having sufficient slack to manage the thermal contraction which occurs when the cable is cooled in the thermally insulated pipe.
Preferably, removal of the spacers is to be performed immediately before a process of housing the cores into the thermally insulated pipe. Usually, the thermally insulated pipe has a structure such that a vacuum thermal insulation layer is formed between an outer pipe and an inner pipe, and the stranded cores are housed in the inner pipe. When the inner pipe is formed from stainless steel, the stainless steel plate covers the outer circumferential surfaces of strands of the cores. Joints between metal plates are sequentially welded by means of a welder. The spacers are removed before the cable cores are introduced into the welder. When the inner pipe is formed from copper or aluminum, in some cases metal is extruded to the outer circumferential surface of the cable cores by means of an extruder. In this case, the spacers are removed before the cables are introduced into the extruder.
Removal of the spacers is performed easily by drawing the spacers from grooves between the stranded cores. For instance, the spacers are drawn sideways of a line and taken up immediately before the cores are introduced into the welder or extruder.
Preferable material for the spacer is a substance having flexibility and strength across the thickness thereof. More specifically, the spacer material includes fluororesin-based material, vinyl-based material, rubber-based material, paper-based material, and felt-based material.
A suitable geometry of the spacer is the shape of an elongated tape. Particularly, the thickness of the spacer preferably satisfies the following requirements.
(1) An amount of required slack A0xe2x89xa6an amount of design slack A1,
where the amount of required slack A0 is expressed by a rate of thermal contraction of the core ({(the length of one pitch of the core after contraction/the length of one pitch of the core before contraction)xe2x88x921}100), and the amount of design slack A1 is expressed by {(L1/L2)xe2x88x921}100, provided that the length of one pitch of the cores having the spacers is taken as L1 and the length of one pitch of cores not having spacers is taken as L2; and
(2) a diameter of an enveloping circlexe2x89xa6an inner diameter B of the thermally insulated pipe,
where the diameter of the enveloping circle corresponds to the diameter of a circle circumscribing the stranded cores having spacers.
Requirement (1) is for managing the contraction of cores. Selection between full management of a required amount of slack or partial management of the same may be determined by design, as required. Requirement (2) is for preventing damage to the cores, which would otherwise be caused when the cores come into contact with the interior surface of the thermally insulated pipe at the time of machining of the thermally insulated pipe. In consideration of tolerances, the inner diameter B of the thermally insulated pipe is preferably set to the minimum value.